Operating a large electric utility type steam turbine in a steam bypass mode entails the use of bypass valving systems to shunt steam around sections of the turbine whenever the load demand is such that the boiler is producing more steam than is required to support the load. The principal advantage of the bypass mode of operation is that the boiler may be operated at high level of output independently of the turbine's demand for steam which, in turn, is a reflection of demand for electrical energy. Other advantages inherent in a bypass mode of operation include the ability to quickly follow changes in load demand, the ability to more rapidly start the turbine, and the avoidance of boiler tripout upon sudden loss of load.
A problem encountered in the bypass mode of operation, however, and for which a solution has been sought by those skilled in the art, is the potentially damaging increase in temperature which can occur within the turbine sections as a result of rotational loss heating under no-load and low-load operating conditions. This heating effect, also commonly referred to as windage loss heating, is due to the friction between the steam and the turbine rotor blading occurring at or near synchronous speeds, and is pronounced in the bypass mode of operation because of the high back pressure resulting from the bypass steam flow and because of the relatively low flow of steam required to pass through the turbine when it is under very light load. The severity of the problem depends upon the rated power capacity of the turbine; the greater the power capability the higher the turbine temperatures are likely to become during these low load conditions. Windage losses at the exhaust end of the high-pressure (HP) section of a turbine can elevate the temperature to an extent that the turbine structure is subjected to excessive thermal stress, resulting in permanent structural damage.
The problem is accentuated by the fact that, as the turbine takes on more load and therefore more steam from the bypass system, the windage losses will be cut sharply and the turbine actually cooled by the increased steam flow. This sudden reversal of temperature puts a severe and sharp stress upon the turbine metal and may cause permanent deformation or cracking thereof.
With the present trend to larger, more efficient power-generating units, and with heightened interest in the bypass mode of turbine operation, solutions to the problem of rotational loss heating have been eagerly sought. However, an entirely satisfactory solution to the problem has not heretofore been available.
One previous approach to the problem, as exemplified by U.S. Pat. No. 4,132,076 entitled "Feedback Control Method for Controlling the Starting of a Steam Turbine Plant" has been to devise a rather elaborate and complicated feedback control system with which a greater quantity of steam is caused to pass through the high-pressure section of the turbine than through the lower pressure sections. This is accomplished by having a control system in which one subsystem provides control of the bypass and steam admission valves at low and no-load conditions and a second subsystem which provides control at elevated loading. While there is thus provided an acceptable means for dealing with the problem of rotational loss heating, other and simpler methods and apparatus are desired.
Accordingly, it is an object of applicants' invention to provide a simple, satisfactory solution to the problem of rotational loss heating such as may occur in steam turbines during bypass mode operation.